1. Field of the Invention
The present invention relates to a display device for displaying images. In particular, the present invention relates to a display device capable of displaying color images. In addition, the present invention relates to a display device having a large screen.
2. Description of the Related Art
Various types of display devices have been proposed. For instance, CRTs are being very widely used in such as television receivers, monitors of information processing device and the like. Further, in recent years, flat-panel displays such as liquid crystal displays (LCDs), plasma display panels (PDPs), field emission displays (FEDs) and the like are remarkable. Among them, liquid crystal displays have been used as the display devices in place of CRTs more frequently.
Now, in order to display color images, it is required that the three images, each of which consists of the light of each primary color, R(red), G(green), and B(blue) are synthesized. As a method of synthesizing images, a spatial division displaying method, in which each image is displayed by the 2-dimensionally arranged R, G, B dots (pixels), is known. Further, a time division display method, in which each image of R, G, B color is time sequentially displayed is also known, too. In generally used color CRTs and liquid crystal displays are based on the spacial division displaying method because R, G, B pixels are 2-dimensionally arranged on a screen.
The time division displaying can be realized by switching the colors of an over-all display screen synchronized with graded images to be displayed rapidly, using using an RGB color filter. In the time division displaying, it is required to display images with higher speed of 3 times or more compared with the case of spatial division displaying. However, since it is not required to decompose an image into R, G, B pixels, more high definition images can be displayed. As a method of switching colors of display screen, for instance, a method of mechanical rotation of a disc filter which is equally divided into 3 parts of different colors. As an electronic color switching method, a liquid crystal color shutter (LCCS) is proposed by Bos et al. (U.S. Pat. No. 5,387,920). The liquid crystal color shutter comprises two liquid crystal cells and color polarizers interposed before and behind them. By ON/OFF switching of the liquid crystal cells to control polarization of incident light, the wavelength of the light absorption by the polarizers are selected, so that RGB selectable displaying can be carried out. Advantages of the color liquid crystal shutter are that there is no mechanically moving component, and it has good space utility because the area of the display screen and that of the color shutter can be made equal.
Despite the aforementioned advantages of the liquid crystal color shutter, there is a problem that its transmittance of light is low due to use of polarizers. Due to the absorption of one component of polarized light, one half of the incident light is lost. Further, due to making chromatic color from achromatic light and switching of RGB displaying, one third of the incident light is lost. Therefore, even in the case of an ideal liquid crystal color shutter, the transmittance is about 16.7% (1/2 (transmission of polarizers).times.1/3 (RGB displaying)=16.7%).
However, due to the light absorption in the transmissive axis of the polarizers and the light absorption of transparent electrodes being employed in the liquid crystal cell, further loss of light occurs. In reality, the transmittance of the liquid crystal color shutter is such low as about 5 to 8%. In order to secure screen brightness enough to display excellent images, the brightness of the emitting face of like a cathode ray tube is required to be increased. Therefore, several problems occur, for example, reduction of resolution power, deterioration of phosphor, increase of electric power consumption and so on.
Thus, a display device or the like using a conventional liquid crystal color shutter and an important problem that the brightness of the display screen is low due to the light absorption of the liquid crystal color shutter.
Displaying systems of a display device can be roughly divided into a direct-viewing type and a projection type. A display device of the direct-viewing type is that an observer views the displayed images directly. A projection type display device is that an observer views the enlarged images which are projected on a screen. The latter system is further divided into a front-projection type in which images are projected onto a reflective screen from the side of an observer and a rear-projection type in which images are projected onto a transmissive screen from behind the screen.
Such a conventional display device like a cathode ray tube or a liquid crystal display is difficult to make large its displaying screen area. For instance, in order to realize a large displaying area in the case of a CRT, either the CRT itself has to be made large or images displayed on a CRT is enlarged on a screen with the use of a projection optical system. However, making a CRT large involves lengthening of depth of the tube, and the weight is also more heavy. Further, in the case of the CRT, it needs to be evacuated inside the tube in which images are formed by irradiation of an electron beam. This also makes difficulty to enlarge the displaying area of a CRT.
To realize a large display screen area in a liquid crystal display device, in the case of the direct-view type, an array substrate itself is necessary to be made large. On the array substrate, many switching elements such as thin-film transistors and pixel electrodes are disposed in a matrix form. However, it is difficult to produce a very huge numbers of extremely fine semiconductors uniformly on the entire of the large substrate. Therefore, productivity of a liquid crystal display device becomes accordingly very low, resulting in a problem of increase of the cost. On the other hand, a plurality of array substrates can be tiled to realize a large displaying area. However, there is a problem that handling of the boundary of the tiled substrate is very difficult. Still further, it is difficult to obtain uniform displaying quality between the tiled plurality of array substrates. In the case of a PDP also, there is difficulty in manufacturing process caused by making large the screen.
A projection type of display device is easy to make large its screen area compared with various display device of direct-view type . However, it has less versatility compared with display device of direct-view type. For instance, illumination of the room is necessary to dark enough. Further, a projection type of display device is difficult to obtain sufficient brightness and contrast, resulting in a problem of worse displaying ability compared with the direct-view type display devices.